The present invention relates to antennas for the transmission and reception of microwave energy. More particularly, the present invention relates to an improvement to a microwave antenna for reducing undesirable echo reflections to the feed system, which reflections are superposed, delayed upon the desired transmitted and received microwave energy.
In the field of space communications, a microwave antenna is used to transmit and receive many communications channels. One such antenna is the Cassegrainian antenna, which has a large concave main reflector, a smaller convex subreflector placed forward of the main reflector and a feed system, often located centrally in an opening in the main reflector. Radiation from the feed is reflected from the subreflector to the main reflector and is transmitted from the antenna as a narrow microwave beam.
Unfortunately, some radiation transmitted from the feed is also reflected undesirably back to the feed from the subreflector. This undesirable reflection is called an echo, the echo corresponding with an impedance mismatch, in this case between the feed and subreflector. The echo causes, for example, an objectionable intermodulation background noise component in frequency division multiplexed FM communications channels which sharply increases as the antenna size and number of channels is increased. See Bell Telephone Laboratories, Transmission Systems for Communications, 4th Ed., pp. 517-522, 1970.
Heretofore, undesirable echo reflections have been reduced by placing an essentially flat reflecting plate near the subreflector between the subreflector and the feed system to cancel some of the echo at the feed. When the plate reflects radiation to the feed which is equal in amplitude and 180 degrees out of phase at a given frequency with the echo at the feed location from the rest of the subreflector, complete echo cancellation at that frequency is obtained. As the number of communications channels is increased, however, the frequency range over which the sharply increased echo-caused noise can be acceptably cancelled by a flat plate decreases. Furthermore, some communications systems use distinct frequency ranges for simultaneous transmission and reception. Consequently, as the number of channels is increased to take full economic advantage of the antenna, the echo-caused noise in these frequency ranges rises above an acceptable level if a flat plate is employed.
Accordingly, it is an object of the present invention to substantially cancel microwave echo reflections over a wide bandwidth in a microwave antenna.
It is another object of the present invention to substantially eliminate echo-caused channel noise from a Cassegrainian antenna accommodating a large number of communications channels.
It is another object of the present invention to eliminate undesirable echo interference to simultaneously transmitted and received communications channels carried in distinct frequency ranges in a microwave antenna.
Attention is called to the copending application of E. A. Ohm entitled "Antenna with Echo Cancelling Elements," Ser. No. 597,366, filed July 21, 1975, in which there is disclosed a dual frequency echo cancelling structure having a gridded design. According to the disclosure in that application, a frequency sensitive reflecting grid of cylindrical wires is placed parallel to a prior art echo cancelling flat plate located between the feeding means and the subreflector. In this manner, a combined reflection from the grid and the plate suffices to cancel much of the subreflector echo returned to the feed system. It is to be understood that the arrangement disclosed in that copending application is regarded herein as operative for cancellation of echoes in two frequency ranges.
Observations made on such a gridded design, indicate, however, that a more complete echo cancellation may be obtained at more frequencies. Some of the radiation incident upon an echo cancelling grid and plate appears to be scattered in undesired directions and not reflected back to the feed system to assist in echo cancellation. This means that the grid diameter must be larger than would be necessary in the absence of such scattering, and the undesirable results include subreflector blockage, reduction of antenna gain, and increased antenna noise temperature. Furthermore, undesirable stray resonance peaks are observed which when eliminated can improve the echo cancelling properties of that structure.
Therefore, it is a further object of the present invention to accomplish a reduction in size and to reduce stray resonances in a dual frequency echo cancelling structure of the gridded variety.